The present invention pertains to the preparation of particular rare earth oxyhalide phosphor materials activated with terbium and/or thulium which exhibits substantial brightness improvement when excited by x-radiation as well as providing less light scattering and absorption when utilized in x-ray image converter devices.
Conventional rare earth oxyhalide phosphors still experience considerable light scattering and absorption by reason of the crystalline characteristics of these plate-like phosphor particles. While a recrystallization process has been known for some time, as described in U.S. Pat. No. 3,591,516, wherein well-formed crystals of these oxyhalide materials can be grown in particular molten alkali metal halide fluxes, there still remains a need to improve the quality of the phosphor crystals for better performance in x-ray image converter devices. The conventional phosphor material still experiences considerable brightness loss attributable to phosphor particle size which is either too fine or too coarse and further brightness loss occurs in radiographic screens when the thickness of the conventional phosphor layers exceeds thicknesses customarily employed. The brightness loss experienced in the latter regard is especially significant with the conventional phosphors and which can be as much as 20 percent or greater in brightness loss if the phosphor loading exceeds 180 milligrams per square centimeter of surface area in the phosphor layer. The light scattering and self-absorption reduce brightness in the conventional phosphor material since the light cannot escape efficiently from the phosphor layer.
A second undesirable effect experienced with the conventional phosphor materials is that of light scattering in directions other than the direction of photographic film and which is due to the plate-like morphology of these phosphor crystals. More particularly, a conventional phosphor has a length and thickness ratio often exceeding 15:1 causing alignment parallel to the major film axis when deposited to form the phosphor layer in a radiographic screen. This alignment contributes to substantial light scattering parallel to said film with both loss in sharpness of the radiographic emission and a longer effective light path to reach the film which enhances the familiar crossover problem encountered with multi-layer radiographic screens. All of the foregoing light scattering problems with the conventional phosphor materials in x-ray image converter devices would be substantially reduced by preparing these phosphor materials in a manner which decreases the length and thickness ratio of the individual phosphor particles.
Accordingly, it is an important object of the present invention to provide a rare earth oxyhalide phosphor material exhibiting improved brightness as well as less light scattering and absorption from a modification of the phosphor crystalline characteristics.
It is another important object of the invention to provide an improved method of preparing rare earth oxyhalide phosphors exhibiting improved performance in x-ray image converter devices.
Still another important object of the present invention is to provide an improved multilayer x-ray screen construction utilizing the present phosphor materials to provide improvement in image sharpness and a reduced crossover problem.